Image sensor arrays, such as found in digital document scanners and digital cameras, typically comprise a linear array of photosites which raster scan a focused image, or an image bearing document, and convert the set of microscopic image areas viewed by each photosite to image signal charges. Following an integration period the image signal charges are amplified and transferred to a common output line or bus through successively actuated multiplexing transistors.
Currently there are two generally accepted basic technologies for creating such linear arrays of photosites: Charge-coupled devices, or CCD's, and CMOS. In CMOS, the photosites include photodiodes, which output a charge in response to light impinging thereon. In the scanning process, bias and reset charges are applied in a predetermined time sequence during each scan cycle. Certain prior art patents, such as U.S. Pat. No. 5,081,536 assigned to the assignee hereof, disclose two-stage transfer circuits for transferring image signal charges from the photosites in CMOS image sensors.
In practical applications of photosensitive devices, two key sources of noise, which can affect the integrity of the output image signals, are “fixed pattern noise” and “thermal noise.” The first of these types of noise relates to the fact that, within any device, individual photodiodes and sets of circuitry associated with the various photodiodes will have some variation in performance, and the variation in performance among the different sets of circuitry will result in a fixed pattern of noise effecting the signals, resulting in a consistent pattern of distortions in the output signals, depending on which specific set of circuitry a particular subset of the video signals passes through. Thermal noise is created by the fact that the output of a particular set of circuitry is likely to change over time, due to the random thermal movement of electrons in conductors.
U.S. Pat. Nos. 5,105,276 and 5,654,755 describe the principle of using a “dark signal” at some point in the operation of a CCD or CMOS image sensor system, respectively, to isolate noise of various types from a useable signal. A dark signal is, broadly speaking, a signal derived from a photosensor when it is known that no light is impinging on the photosensor. Typically the dark signal is subtracted from a total signal output from a photosensor, thus leaving the pure signal as a remainder.
U.S. Patent Publication Serial No. US 2005/0094222-A1 discloses an image sensor in which dark signals and total signals from a photosensor are, through a system of switches, directed to separate holding capacitors with each readout of the photosensor. During a readout process, the dark signal is subtracted from the total output signal.
U.S. Pat. No. 5,148,268 discloses an overall architecture for a CMOS-based full-color photosensitive chip.
U.S. Pat. No. 5,493,423 discloses the use of a “standby mode” for an amplifier used with a “fat zero” transfer circuit associated with a photosensor.